Jane,
The main contributors to construction cost are
1 - overall size, including porches, decks, garages and unfinished "future" spaces, as well as finished living area.
2 - the type and complexity of that space.
A plan with more "footprint" (large first floor, small second floor) will cost more than the same amount of square feet balanced over both floors.
A steep roof costs more than a lower pitch (OSHA rules & practical issues)
More dormers, corners, bumps and grinds increase the cost.
More high-ticket space (kitchen, bath, built-ins) costs more.
Here's some numbers I use to get clients here started (Greater Boston area) - assumes using a regular General Contractor, no owner sweat equity. Keep in mind that these are starting points. You may hear stories of people building for less, but remember that pride makes people fudge a bit, and that your cousin or your friend may not have really added everything up. These starting numbers also don't include the kind of built-ins and interior work that most people looking at this site are attracted to.
Regular old two story construction - $150-165/sf (for each floor). It's not unusual for a not-so-big house, with the built-ins and interior design you see in the books, to run more like $250/sf or more.
Cathedral ceiling and single floor - $190/sf
Porches - $75/sf
Unfinished "future" space - $75/sf
Decks - $40/sf
Garage - add about $25,000 for a 24x24 "normal" two car. Don't forget that the "bonus" space above is not free - or dormers, or an elaborate roof...
More than the usual windows - add $350 each for average windows, visit your window dealer...
Master Baths and Kitchens can very wildly with the design. Visit some cabinet dealers, talk to your builder...
Built-ins and upgraded interior trim - talk to a builder, finished carpenter or cabinet-maker.
I would also add to the basic number for upgraded staircases, more than one staircase, very elaborate roof design.
A deep footprint also adds cost (30 feet at the smallest dimension costs more than 26 feet at the smallest dimension, for the same total square feet).
I've found that the only way to really know is to choose your builder and work with him or her to price a design as you go. But, I know it's hard to know where to start. I hope this helps.
Wendy Welton, AIA
Art Form Architecture, Inc.
www.artform.biz"Our Creativity, Your Dreams"